If anything, I would assume a combination of Postfix/Cyrus - like their default set in OSX Server.
IMHO, I think Apple would have a challenge trying to create an Exchange-like solution in the OSX Server environment, simply because of the all-in-one solution that Exchange tends to be. Calendars and e-mail are stored in the same mailbox store, are they not? iCal Server and Cyrus keep separate information stores, and are even managed slightly differently in Open Directory. Just a few thoughts, I suppose.
